1. Field of the Invention
The present invention relates to an optical tomographic imaging method and apparatus for obtaining a tomographic image of a measuring object by OCT (optical coherence tomography) measurement, and a light control unit applicable to the optical tomographic imaging apparatus.
2. Description of the Related Art
An optical tomographic image obtaining system using OCT measurement is sometimes used to obtain an optical tomographic image of a living tissue. In the optical tomographic image obtaining system, a low coherence light beam outputted from the light source is split into measuring and reference beams, and the measuring beam is irradiated onto a measuring object, then the reflected beam from the measuring object or backscattered light when the measuring beam is irradiated thereon is combined with the reference beam, and an optical tomographic image is obtained based on the intensity of the interference beam between the reflected beam and the reference beam. Hereinafter, reflected beam from a measuring object and backscattered light are collectively referred to as the “reflected beam”.
The OCT measurement is largely categorized into TD-OCT (Time Domain OCT) measurement and FD (Fourier Domain)-OCT measurement. The TD-OCT measurement is a method for obtaining a reflected beam intensity distribution corresponding to a position in the depth direction (depth position) of a measuring object by measuring interference beam intensity while changing the optical path length of the reference light.
The FD-OCT measurement is a method for obtaining a reflected light intensity distribution corresponding to a depth position of a measuring object by measuring interference beam intensity with respect to each spectral component of the beam without changing the optical path length of the reference beam, and performing frequency analysis, typically a Fourier transform, on the obtained spectral interference intensity signals using a computer. The FD-OCT does not require the mechanical scanning used in TD-OCT, so that it has been drawing wide attention as a method that allows high speed measurement.
Typical systems that use FD-OCT measurement are SD-OCT (Spectral Domain OCT) system and SS-OCT (Swept Source OCT) system. The SD-OCT system uses a broadband and low coherence light beam, such as SLD (Super Luminescence Diode), ASE (Amplified Spontaneous Emission), or white light beam, as the light source, and forms an optical tomographic image in the following manner. The broadband and low coherence light beam is split into measuring and reference beams using Michelson interferometer or the like and the measuring beam is irradiated onto a measuring object, then a reflected beam from the measuring object when the measuring beam is irradiated thereon is caused to interfere with the reference beam and the interference beam is broken down into frequency components using a spectroscopic device, thereafter the intensity of the interference beam with respect to each frequency component is measured using a detector array including elements, such as photodiodes, disposed in an array and an optical tomographic image is formed by performing Fourier transform on the obtained spectral interference signals using a computer.
In the mean time, the SS-OCT system uses a laser that temporally sweeps the optical frequency, in which the reflected beam is caused to interfere with the reference beam at each wavelength, then the temporal waveform of the signal corresponding to the temporal change in the optical frequency is measured and an optical tomographic image is formed by performing Fourier transform on the obtained spectral interference signals using a computer.
In various types of OCT measurements described above, a broader wavelength range of the light source and increased number of corresponding data points are desired in order to improve spatial resolution and to obtain a high quality image. The conventional Fourier transform method requires a light source having a continuous spectrum. As for the light source, a semiconductor light source, such as a small and inexpensive superluminescence diode (SLD), semiconductor optical amplifier, or the like, is desirable. But, the gain bandwidths of these devices are limited due to their medium characteristics, so that it is difficult to realize a continuous bandwidth exceeding 100 nm with a single device.
Consequently, a method for broadening the wavelength range by combining light beams outputted from a plurality of light sources are disclosed, as described, for example, in Japanese Unexamined Patent Publication No. 2002-214125. As a broadband spectrum light source, the method uses a plurality of light sources that output light beams having different spectral ranges with each other, and light beams outputted from respective light sources are combined using an optical coupler to output a single wave beam.
For the SD-OCT measurement, a method for forming a continuous spectrum by combining light beams from a plurality of gain media, each having a overlapping wavelength range with each other, is disclosed in Japanese Unexamined Patent Publication No. 2001-264246. As for the method of forming a continuous spectrum through wavelength combination for SS-OCT, a structure including a plurality of wavelength scanning light sources, each having a gain medium and a wavelength selection element is disclosed in Japanese Unexamined Patent Publication No. 2006-047264. Further, U.S. Pat. No. 6,665,320 discloses a structure that simultaneously controls light beams from a plurality of gain media using a single wavelength selection element.
With regard to increasing the data points, the interference beams are generally detected with respect to each wavelength using a detector array including elements, such as photodiodes, disposed in an array, so that the number of data points is limited by the number of elements of the detector array in the SD-OCT system. At present, it is not desirable to increase the number of elements of the detector array for increasing the number of data points, since such increase would result in cost increase, decreased manufacturability, reduced measuring rate, and the like. On the other hand, in the SS-OCT system, in order to increase the number of data points, for example, it is just necessary to increase the sampling frequency of the circuit that converts an optical current signal from the detector to a digital value if the frequency sweep period of the light source is assumed to be constant, so that it may be realized easily at low cost with a high measuring rate.
When combining a plurality of light sources in order to obtain high spatial resolution as described above, if the light beams outputted from the plurality of light sources are combined using, for example, a coupler with a branching ratio of 50:50, the light utilization efficiency is degraded since the total output is reduced to a half by the coupler. Another method is to combine the light beams using a polarization beam splitter, but this method allows beam combination of up to only two beams, thereby bandwidth broadening is limited.
Further, where light beams from a plurality of light sources are combined and used, the conventional SS-OCT system poses a problem that, when light beams having different wavelengths are outputted from a plurality of light sources and irradiated onto a measuring object at the same time, the interference information provided by the plurality of light beams is mixed up and unable to be detected since the detector of the system includes only a single element.
For this reason, in the systems described in Japanese Unexamined Patent Publication No. 2006-047264, and U.S. Pat. No. 6,665,320, a configuration is adopted in which only a single wavelength is inputted to the detector at a time by controlling the light source or using a switching element. Such method, however, poses a problem that the measuring rate is reduced since it takes time to irradiate all of the wavelengths of the measuring beam, though it may provide a broadband beam as the measuring beam.
Therefore, in the OCT measurement, if it is possible to use a plurality of light beams having different wavelengths at the same time and to obtain interference information with respect to each of the light beams at the same time by separating the interference information provided by the plurality of light beams, a high resolution measurement may be performed with a high measurement rate. As such, an optical tomographic imaging method and apparatus having such capabilities have been demanded.
The present invention has been developed in view of the circumstances described above, and it is an object of the present invention to provide an optical tomographic imaging method and apparatus capable of obtaining a high resolution tomographic image rapidly.